When `makemigrations` happens, `B` won't include the `a` FK so subsequent data migrations won't be able to query by `a`

0001_initial.py

# Generated by Django 2.1.dev20180228190652 on 2018-03-01 11:08

from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    initial = True

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='B',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
            ],
            options={
                'db_table': 'test_b',
                'managed': False,
            },
        ),
        migrations.CreateModel(
            name='A',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
            ],
            options={
                'db_table': 'test_a',
            },
        ),
        migrations.CreateModel(
            name='C',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('a', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='+', to='whee.A')),
            ],
            options={
                'db_table': 'test_c',
            },
        ),
    ]

0002_data_migrate_testing.py

# Generated by Django 2.1.dev20180228190652 on 2018-03-01 11:51

from django.db import migrations


def forwards(apps, schema_editor):
    B = apps.get_model('whee', 'B')
    B.objects.filter(a=1) # if this doesn't crash, yay, B has the field!

def backwards(apps, schema_editor):
    return

class Migration(migrations.Migration):

    dependencies = [
        ('whee', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(forwards, backwards),
    ]

_unmanaged_models_no_fks.py

from django.db import models

class A(models.Model):
    class Meta:
        #managed = False
        db_table = "test_a"


class B(models.Model):
    a = models.ForeignKey(A, related_name="+", on_delete=models.CASCADE)

    class Meta:
        managed = False
        db_table = "test_b"


class C(models.Model):
    a = models.ForeignKey(A, related_name="+", on_delete=models.CASCADE)

    class Meta:
        db_table = "test_c"

Commenting out the lines:

            # Don't add operations which modify the database for unmanaged models
            if not model_opts.managed:
                continue

from https://github.com/django/django/blob/75527c0f8360ae3555fcf67ce19b4cb910b69b9d/django/db/migrations/autodetector.py#L570-L572

puts the FK into B, but does so differently depending on whether A is managed = True or managed = False

• if A.Meta.managed = True then there's a CreateModel for B (containing just the id field) followed by an AddField adding a to B.
• if A.Meta.managed = False then there's just a CreateModel for B which includes both the id and the a fields at once.